Reproduction in marsupials is distinguished by a short gestation which can be extended by a period of embryonic diapause. These mammals offer a unique opportunity to study the control of embryonic development. Their placental attachment is a simple interdigitation of maternal and fetal tissue, but the placenta is apparently an endocrine organ. The resumption of embryonic growth after delay is a complex interaction of blastocyst and uterine secretions. Steroid hormones have been implicated in the initiation of secretory activity and blastocyst development Detailed information is needed on the development of the embryo, on the interactions between embryo and uterus, and the metabolic activity of the embryo at the two critical stages, namely, resumption of blastocyst growth and the formation of the yolk sac placenta. This information will be obtained: 1. describing the morphology of the embryo of the tammar wallaby, Macropus eugenii, from early cleavage stages through bastocyst development to the full term embryo using classical histology techniques and photomicrography. 2. metabolic studies on single blastocysts to determine changes occurring between dormancy and resumed development; and 3. steroid assays of dormant and active blastocysts and the measurement of subsequent hormone production by the yolk sac placenta. Since the hormonal milieu of pregnant marsupials is largely unknown, measurements will all be made of the maternal steroid hormone levels during both estrous cycles and gestation in order to correlate these findings with the changes in the blastocyst, corpus luteum and endometrium throughout gestation.